Short synthesis of pyridine-based pharmaceutical intermediates

ABSTRACT

A method of making a compound of Formula VI:  
                 
 
wherein Tr is a triphenyl group; R 1 , R 2  and R 3  are each independently selected from the group consisting of H, C1-C4 alkyl, C1-C4 alkoxy, aryl, heteroaryl, and arylalkyl; R 4  is C2-C6 alkyl, and R 5  and R 6  are each independently H or C1-C4 alkyl, involves the step of reacting a compound of Formula V:  
                 
with Tr-OH to produce a compound of Formula VI. The compounds of Formula VI are useful as intermediates in the manufacture of antibiotic agents. Methods of making compounds of Formula V, and intermediates made or used in the foregoing methods, are also described.

FIELD OF THE INVENTION

The present invention concerns methods for the synthesis of pyridine-based compounds, which compounds are useful as intermediates for the manufacture of pharmaceutical compounds.

BACKGROUND OF THE INVENTION

Over the past three decades a large variety of antibiotics have become available for clinical use. Unfortunately, the wide-spread use of these antibiotics has caused a rapid increase in the number of bacterial strains that are resistant to the currently available antibiotics.

S. Hecker et al., PCT Application WO 01/21623 (published 29 Mar. 2001), describes 7-acylamino-3-heteroarylthio-3-cephem carboxylic acid antibiotics and prodrugs thereof. The compounds described therein are active as antibiotics against a wide spectrum of organisms including organisms which are resistant to beta-lactam antibiotics. However, the compounds described therein are complicated, and require the synthesis of a variety of separate groups. One group which must be synthesized to make these compounds is the C3 side-chain, an intermediate for which is illustrated on page 51 therein as follows:

However, the synthesis of such C3 side chain groups as set forth in S. Hecker et al. requires in excess of 6 steps (see pages 49-52 therein). Accordingly, there is a need for new ways to make the intermediates used to make the antibiotic compounds described in S. Hecker et al.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the present invention is a method of making a compound of Formula VI:

wherein:

-   -   Tr is a triphenyl group;     -   R₁, R₂ and R₃ are each independently selected from the group         consisting of H, C1-C4 alkyl, C1-C4 alkoxy, aryl, heteroaryl,         and arylalkyl;     -   R₄ is C2-C6 alkyl, and     -   R₅ and R₆ are each independently H or C1-C4 alkyl, comprising:     -   reacting a compound of Formula V:         with Tr-OH to produce a compound of Formula VI.

A second aspect of the present invention is a method of making a compound of Formula V as described above, the method comprising reacting a compound of Formula IV:

where tBu is tert-Butyl or other suitable leaving group, with R₆R₅NR₄SSR₄NR₅R₆ in the presence of a strong amide base to produce a compound of Formula V.

A third aspect of the present invention is a method of a compound of Formula IV as described above, comprising reacting a compound of Formula III:

wherein X is halogen, with sodium tert-butylthiolate or potassium tert-butylthiolate to produce a compound of Formula IV.

A fourth aspect of the present invention is a compound of Formula VI:

wherein:

-   -   Tr is a triphenyl group;     -   R₁, R₂ and R₃ are each independently selected from the group         consisting of H, C1-C4 alkyl, C1-C4 alkoxy, aryl, heteroaryl,         and arylalkyl;     -   R₄ is C2-C6 alkyl, and     -   R₅ and R₆ are each independently H or C₁-C4 alkyl;     -   subject to the proviso that (i) R₁, R₂ and R₃ are not all         simultaneously H, or (ii) R₄ is not C2, or (iii) R₅ and R₆ are         not simultaneously H.

A further aspect of the present invention is a compound of Formula V:

wherein:

-   -   tBu is tert-butyl, or other suitable leaving group;     -   R₁, R₂ and R₃ are each independently selected from the group         consisting of H, C1-C4 alkyl, C1-C4 alkoxy, aryl, heteroaryl,         and arylalkyl;     -   R₄ is C2-C6 alkyl, and     -   R₅ and R₆ are each independently H or C1-C4 alkyl.

Compounds of Formula VI above are useful as intermediates in the manufacture of antibiotic compounds.

Compounds of Formula V above are useful as intermediates in the manufacture of compounds of Formula VI.

The foregoing and other objects and aspects of the present invention are explained in greater detail in the specification set forth below.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

“Alkyl” as used herein refers to linear or branched alkyl, preferably linear alkyl, including but not limited to methyl, ethyl, propyl, and butyl (Bu).

“Halo” as used herein refers to any suitable halogen group, such as fluoro, chloro, bromo, or iodo.

“Aryl” as used herein refers to any suitable aromatic group, such as phenyl, which aromatic group may be substituted or unsubstituted.

“Arylalkyl” as used herein refers to any suitable aryl group covalently coupled to an alkyl group, such as benzyl.

“Triphenyl” or “Tr” groups as used herein may be unsubstituted or substituted one or more times by additional groups such as C1-C4 alkyl, C1-C4 alkyloxy, or halo. Para substitutions are preferred, but substitutions may be of any number from 1 to 5 and in any position, with mono or di substitutions preferred.

As noted above, a first aspect of the present invention is a method of making a compound of Formula VI:

wherein:

-   -   Tr is a triphenyl group;     -   R₁, R₂ and R₃ are each independently H C1-C4 alkyl, C₁-C4         alkoxy, aryl, heteroaryl, or arylalkyl (preferably H),     -   R₄ is C2-C6 alkyl (preferably C2), and     -   R₅ and R₆ are each independently H or C1-C4 alkyl (preferably         H).

The method comprises reacting a compound of Formula V:

with Tr—OH to produce a compound of Formula VI. The reacting step is may be carried out as a one-pot, two step reacting step. The reaction step is preferably carried out in the presence of a strong organic acid, examples including but not limited to methanesulfonic acid and arylsulfonic acid (e.g., paratoluene sulfonic acid). The reacting step is typically carried out in a polar solvent such as acetic acid, which solvent is preferably non-aqueous, and may be carried out at any suitable temperature such as at room temperature.

Compounds of Formula VI are useful, among other things, as C-3 side chain intermediates useful for the production of 7-acylamino-3-heterarylthio-3-cephem carboxylic acid antibiotics, and prodrugs thereof, as shown in S. Hecker et al., PCT Application WO 01/21623 (29 Mar. 2001) (see pages 49-50). Particularly organisms for which the compounds of the invention may be used as antibiotics include but are not limited to Staphylococcus aureus, Enterobacteriaceae, and Pseudomonas. The compounds may be used in vivo as a pharmaceutical agent by (for example), oral, parenteral, or topical administration, or may be used in vitro, for example as a topical or surface antibiotic.

A compound of Formula V above may be produced by reacting a compound of Formula IV:

with R₆R₅NR₄SSR₄NR₅R₆ (which may be produced in accordance with known techniques), preferably in the presence of a strong amide base, to produce a compound of Formula V. This reacting step may be carried out in any suitable solvent, typically an etherial solvent such as dialkyl ether (e.g., diethyl ether), dimethoxyethane, tetrahydrofuran, or mixtures thereof. Any suitable strong amide base may be used, such as lithium, sodium, and potassium amide bases. Any suitable amide may be used, such as a dialkyl amide (e.g., diethyl amide). The temperature at which the reacting step is carried out is not critical, but is preferably less than room temperature (e.g., from −80 to −20 or even 0 degrees centigrade).

The compound of Formula IV above may be produced by reacting a compound of Formula III:

wherein X is halogen, with sodium tert-butylthiolate or potassium tert-buthylthiolate to produce a compound of Formula IV. This reacting step may be carried out in any suitable solvent, preferably nonaqueous, such as a polar aprotic solvent (e.g., dimethylformamide and/or dimethylsulfoxide). The reacting step may be carried out at any suitable temperature, such as from 20 to 120 or 130 degrees centigrade. Compounds of Formula III are known or may be prepared in accordance with known techniques.

The present invention is explained in greater detail in the following non-limiting examples.

EXAMPLES 1-4

Examples 1 to 4 below illustrate the set of reactions shown in Scheme 2 below.

Example 1 Cystamine (2)

H₂NCH₂CH₂SSCH₂CH₂NH₂

To a suspension of cystamine dihydrochloride (1) (10.0 g, 44.4 mmol) in anhydrous methanol (20 mL) was added NaOMe (20 mL, 88.8 mmol, 25 wt. % solution in methanol) slowly. The mixture was stirred for 0.5 h and then filtered through a fritted funnel. The solvent was removed in vacuo without heating (caution: heating the solution can cause decomposition of cystamine). The residue was dissolved in diethyl ether and then filtered. The filtrate was concentrated in vacuo and the residue was bulb-to-bulb distilled (120° C., 0.5 mmHg) to afford 4.8 g (70%) of the desired product 2 as a colorless liquid. The oil was dissolved in DME and used directly in the next step.

Example 2 3-tert-Butylsulfanyl-2-methylpyridine (4)

To a solution of 3-bromo-2-methyl-pyridine (3) (15.0 g, 87.1 mmol) in DMF (100 mL) was added sodium tert-butylthiolate (11.7 g, 104.5 mmol) under N₂. The mixture was heated to 130° C. for 3 h. After cooling, it was poured into EtOAc (200 mL) and washed with water (3×100 mL). The organic layer was dried over MgSO₄ and the solvent was removed in vacuo. The residue was bulb-to-bulb distilled (90° C., 0.5 mmHg) to afford 13.9 g (88%) of the desired product as a colorless liquid. FTIR (thin film) 2962, 1559, 1419, 1364, 1168 cm⁻¹. ¹H NMR (300 MHz, CDCl₃) δ 1.30 (s, 9 H), 2.77 (s, 3 H), 7.10 (t, J=4.7 Hz, 1 H), 7.79 (d, J=7.7 Hz, 1 H), 8.47 (d, J=4.4 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃) δ 24.3, 30.8 (isomer), 31.0, 47.7, 120.9, 123.0 (isomer), 128.5, 144.7 (isomer), 146.0, 149.0, 156.3 (isomer), 163.7. HRMS calcd. for C₁₀H₁₆NS (M+H)⁺: 182.1003. Found: 182.1006 (M+H)⁺.

Example 3 2-[(3-tert-Butylsulfanyl)pyridin-2-ylmethylsulfanyl]ethylamine (5)

To a three-necked flask equipped with a mechanical stirrer was added THF (200 mL), n-BuLi (17.4 mL, 43.5 mmol, 2.5 M in hexane) and isopropylamine (5.7 mL, 43.5 mmol) at −78° C. After 30 minutes, compound 4 (7.1 g in 20 mL THF, 39.5 mmol) was added dropwise. After stirring for 15 minutes, cystamine (2) (7.2 g in 20 mL DME, 47.3 mmol) was added in one portion. The mixture was warmed to rt slowly and stirred overnight. It was poured into water (100 mL) and extracted with EtOAc (3×100 mL).

The combined organic layers were dried over MgSO₄, and the solvent was removed in vacuo. The residue was bulb-to-bulb distilled (150° C., 0.5 mmHg) to afford 7.5 g (75%) of the desired product as a brown liquid. FTIR (thin film) 3357, 2961, 1560, 1458, 1364 cm⁻¹. ¹H NMR (300 MHz, CDCl₃) δ 1.31 (s, 9 H), 2.66 (t, J=6.2 Hz, 2 H), 2.88 (t, J=6.2 Hz, 2 H), 4.20 (s, 2 H), 7.16 (dd, J=7.7, 4.7 Hz, 1 H), 7.83 (d, J=7.8 Hz, 1 H), 8.53 (d, J=4.6 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃) δ 31.2, 36.2, 36.3, 41.3, 47.9, 121.9, 128.6, 146.1, 149.3, 163.6. HRMS calcd. for. C₁₂H₂₁N₂S₂ (M+H)⁺: 257.1146. Found: 257.1143 (M+H)⁺.

EXAMPLE 4 2-[3-(Triphenylsulfanyl)pyridin-2-ylmethylsulfanyl]ethylamine (6)

To an argon purged flask was added compound 5 (0.36 g, 1.40 mmol), methanesulfonic acid (2 mL) and acetic acid (4 mL). The mixture was heated to reflux for 20 h and then the solvent was removed in vacuo. The residue was dissolved in dichloromethame (20 mL) and triphenylmethanol (0.44 g, 1.68 mmol) was added. After stirring at rt for 1 h, the mixture was poured into aqueous NaHCO₃ with caution and then extracted with dichloromethane (2×50 mL). The combined organic layers were dried over MgSO₄ and the solvent was removed in vacuo. The residue was purified by radial PLC (methanol) to afford 0.52 g (85%) of the desired product as a thick white oil. FTIR (thin film) 3363, 3055, 1599, 1489 cm⁻¹. ¹H NMR (300 MHz, CDCl₃) δ 1.93 (br s, 2 H), 2.51 (m, 2 H), 2.75 (m, 2 H), 3.48 (s, 2 H), 6.70 (m, 1 H), 7.12-7.37 (m, 16 H), 8.19 (d, J=4.1 Hz, 1 H). ¹³C NMR (75 MHz, CDCl₃) δ 31.1, 35.5, 41.0, 71.6, 121.4, 126.9. 127.0, 127.2, 127.8, 129.8, 130.7, 141.6, 143.6, 147.4, 161.5. HRMS calcd. for C₂₇H₂₆N₂S₂: 443.1616 (M+H)⁺. Found: 443.1618 (M+H)⁺.

The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein. 

1-19. (cancelled)
 20. A compound of Formula VI:

wherein: Tr is a triphenyl group; R₁, R₂ and R₃ are each independently selected from the group consisting of H, C1-C4 alkyl, C1-C4 alkoxy, aryl, heteroaryl, and arylalkyl; R₄ is C2-C6 alkyl, and R₅ and R₆ are each independently H or C1-C4 alkyl; subject to the proviso that (i) R₁, R₂ and R₃ are not all simultaneously H, or (ii) R₄ is not C2, or (iii) R₅ and R₆ are not simultaneously H.
 21. A compound according to claim 20, wherein R₁ R₂ and R₃ are H.
 22. A compound according to claim 20, wherein R₄ is C2 alkyl.
 23. A compound according to claim 20, wherein R₅ and R₆ are both H.
 24. A compound of Formula V:

wherein: tBu is tert-butyl; R₁, R₂ and R₃ are each independently selected from the group consisting of H, C1-C4 alkyl, C1-C4 alkoxy, aryl, heteroaryl, and arylalkyl; R₄ is C2-C6 alkyl, and R₅ and R₆ are each independently H or C1-C4 alkyl.
 25. A compound according to claim 24, wherein R₁ R₂ and R₃ are H.
 26. A compound according to claim 24, wherein R₄ is C2 alkyl.
 27. A compound according to claim 24, wherein R₅ and R₆ are both H.
 28. (Cancelled). 